The present invention relates to a pneumatic radial ply runflat tire whose runflat handling is improved by providing a means for stiffening the tread and its underlying support structure. The increased rigidity improves the tire""s runflat capability by resisting the tendency of the center of the tread to buckle upward during operation under conditions of unpressurized or underpressurized operation. During high-speed operation, the increased rigidity resists upward buckling of the center part of the tread and its underlying structure thereby enhancing the tread""s ground contact and lateral grip.
Various methods have been devised for enabling the safe, continued operation of unpressurized or underpressurized vehicle tires with the intent of minimizing further damage to the uninflated tire and without simultaneously compromising vehicle handling over a distance from the place where the tire lost its pressure to a place desired by the driver, such as a service station where the tire can be changed. Loss of tire pressure can result from a variety of causes, including puncture by a foreign object such as a nail or other sharp object piercing the pneumatic tire installed on a vehicle.
Pneumatic tires designed for sustained operation under conditions of unpressurization or underpressurization are also called runflat tires, as they are capable of being driven in the uninflated or xe2x80x9cflatxe2x80x9d condition. The conventional pneumatic tire, when operated without inflation, collapses upon itself, its sidewalls buckling outward in the region where the tread contacts the ground, when supporting a vehicle load. In general, the term xe2x80x9crunflatxe2x80x9d means that the tire structure alone has sufficient rigidity and strength to support the vehicle load when the tire is operated in the uninflated condition such that the sidewall and internal surfaces of the tire do not collapse or buckle onto themselves, i.e., without recourse to incorporation of other internal supporting structures and devices to prevent the tire from collapsing.
An example of a runflat tire design is described in U.S. Pat. No. 4,111,249, entitled the xe2x80x9cBanded Tire,xe2x80x9d in which a hoop or annular band approximately as wide as the tread is circumferentially deployed beneath the tread. The hoop in combination with the rest of the tire structure could support the vehicle weight in the uninflated condition. This prior art banded tire is disclosed as being able to actually induce tension the ply cords even in the uninflated condition.
Numerous methods have been used to achieve workable runflat tire designs. Generally, such tires incorporate sidewall designs that are thicker and/or stiffer, so that the tire""s load can be carried by an uninflated tire with minimum adverse effects upon the tire itself and upon vehicle handling until such reasonable time as the tire can be repaired or replaced. The methods used in sidewall stiffening include the incorporation of circumferentially disposed inserts in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire usually having the lowest resistance to deformation under vertical loading. In such runflat tire designs, the thickness of the sidewalls increases and decreases again to form a crescent shaped section between the bead and the tread. The reinforced sidewalls of such tires, when operated in the uninflated condition, experience a net compressive load. The outer portions of the reinforced sidewalls are in tension due to bending deformation which deflects the sidewalls outward or apart from one another in the regions of the sidewall adjacent to the ground-contacting portion of the tread. The inner portions of such reinforced sidewalls, in the region near where the tread contacts the ground, tend to be in compression during runflat operation.
Due to the large amounts of rubber required to stiffen the sidewall members, heat buildup due to flexure of the sidewalls is a major factor in tire failure, especially when the uninflated tire is operated for prolonged periods at high speeds.
A Goodyear patent, U.S. Pat. No. 5,368,082 (""082) disclosed a low aspect runflat pneumatic radial ply tire, which employs special sidewall inserts to improve stiffness. Approximately six additional pounds (2.72 kilograms) of weight per tire was required to support 800 pounds (lb.) [360 kilograms] with this uninflated tire.
This earlier invention, although superior to prior attempts at runflat tire design, still imposed a weight penalty that could be offset by the elimination of a spare tire and the tire jack. However, this weight penalty was even more problematic when the engineers attempted to build high-aspect-ratio tires for large luxury touring sedans. These taller sidewalled tires, having aspect ratios in the 55% to 65% range or greater, means that the sidewall bending stresses are greater than that of the earlier low-aspect-ratio runflat tires disclosed in the ""082 patent. Thus the sidewalls of high profile tires had to be stiffened to the point of compromising ride characteristics. Luxury vehicle owners generally do not wish to sacrifice ride quality for runflat capability. The engineering requirements for runflat tire design require that there be no loss in ride or performance. In the very stiff suspension performance type vehicle, such as sport cars and various sport/utility vehicles, the ability to provide such runflat tires is relatively straightforward compared to providing similar runflat tires for luxury sedans which require softer ride characteristics. Light trucks and sport utility vehicles, although not as sensitive to ride performance, provide a runflat tire market that ranges from accepting a stiffer ride to demanding the softer luxury type ride.
French Patent Application 78 13956, Publication No. 2,245,334, discloses a tire having a pair of reinforcing plies 23.2, a lower reinforcing ply 31, an intermediate layer of rubber 30 and a lenticular section 20 contained in each xe2x80x9cwall 14xe2x80x9d. U.S. Pat. No. 4,262,726 discloses a tire having a soft rubber cushion 29 between a carcass overlay 20 and a nearest belt ply 15, to permit unhampered reorientation of the cords of these two components during molding.
In general, runflat tire design is based on the installation of one or more inserts inside each sidewall flex area. The inserts in each sidewall, in combination with the plies, add rigidity to the sidewalls in the absence of air pressure during runflat operation. While the high resistance to compression deflection of the inserts provides the necessary resistance to the collapse of the uninflated loaded tire, this method has several drawbacks which include the above mentioned increase in tire weight and, especially during runflat operation, heat buildup in the insert reinforcements of the sidewalls.
Moreover, during runflat operation, the thick reinforced sidewalls tend to transmit bending stresses to the portion of the tread that contacts the ground. The result is that the central portion of the tread tends to buckle upwards from the ground. The upward buckle reduces the ground contact in the tread""s central region, resulting in compromised vehicle handling as well as reduced runflat tread life.
Upward buckling of the tread has adverse effects upon vehicle handling in the runflat mode. Also, the cyclical flexure of the tread during runflat operation tends to cause excessive heating of the tread material, especially during high-speed operation. The excessive heating leads to deterioration of the tire structure in the region of the tread and thereby reduces the runflat tire""s operating life in the runflat mode. A hypothetically perfect runflat tire would be able to maintain the central portion of its tread in the same degree of road contact during runflat operation as during fully inflated operation. Then, the high speed handling of the tire would be improved because of the increased lateral grip of the tire.
It is an object of the present invention to provide a runflat radial tire as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
One object of the present invention is to provide a runflat radial tire having a laterally stiffened tread that resists upward buckling during runflat operation.
Another object of the present invention is to provide a runflat radial tire having a circumferentially stiffened tread that resists upward buckling during runflat operation.
Still another object of the present invention is to provide a runflat radial tire having good high-speed runflat handling characteristics by employing lateral and circumferential stiffening of the tread so as to resist buckling, and consequent loss of ground contact, during runflat operation.
Yet another object of the present invention is to provide a runflat radial tire having good runflat service life as a result of reduced flexure of the tread during runflat operation.
The present invention relates to a pneumatic radial ply runflat tire having a tread, a casing with two insertreinforced sidewalls, two inextensible annular beads, a radial ply structure, a belt structure located radially between the tread and the radial ply structure, a fabric underlay characterized by being deployed radially inward of the belt structure and radially outward of the radial ply structure, and an elastomer insert characterized by being circumferentially deployed radially inward of the belt structure and radially outward of the fabric underlay and the underlying radial plies. The high-modulus reinforcing cords of the fabric underlay are circumferentially aligned, and they are capable of supporting tensile loads. The elastomeric insert, located beneath the belt structure, extends across about 40 percent to about 100 percent and preferably about 50 percent to about 90 percent of the width of the belt structure and is centered upon the tire""s equatorial plane. The insert acts to separate the belt structure from the fabric underlay and the radial ply structures, thereby giving mechanical advantage to the compression-load-bearing belt structure which, in concert with the fabric underlay, contributes to the tread""s circumferential rigidity, the insert, also acting in concert with the radial ply structure, contributes to the tread""s lateral rigidity. At least one of the radial plies might be reinforced with inextensible metal cords.
In one embodiment of the invention, the pneumatic runflat radial tire has low-aspect-ratio (in the range of about 30% to about 60%) design. Such an embodiment has potential for runflat use in high-performance sports type vehicles or light trucks. This low-aspect-ratio, radial ply, runflat pneumatic tire contains two radial belts, a fabric underlay deployed circumferentially about the tire, radially outward of the ply structure and radially inward of the belt structure, and an elastomeric insert circumferentially deployed radially inward of the belt structure and radially outward of the fabric underlay. The fabric underlay has reinforcing cords that are oriented circumferentially, more or less parallel to the tire""s equatorial axis. The combination of the insert and the tensile-stress-bearing fabric underlay contributes to circumferential rigidity of the tread, and the combination of the insert and the radial plies contributes to lateral rigidity of the tread. The enhanced rigidity of the tread resists upward buckling of the central portion of the tread during runflat operation, thereby maintaining good tread-to-road contact and providing improved high-speed runflat vehicle handling. The enhanced rigidity of the tread also reduces the runflat cyclical flexure that can cause heat buildup and consequent tire deterioration.
A second embodiment of this invention relates to a high-aspect-ratio (in the range of about 50% to about 80%) version for a high-profile tire. An example of a use of the high-profile embodiment would be in luxury-type vehicles, high-standing sport-utility vehicles, and some light trucks. During runflat operation of the second embodiment, the enhanced lateral and circumferential rigidity of the tread contributes improved runflat handling and stability as well as a longer runflat service life.
A third embodiment of this invention relates to a radial ply runflat tire having at least one radial ply which is reinforced by essentially inextensible cords, such as steel, and in which the aspect ratio of the tire can be between the maximum and minimum values of aspect ratios for runflat tires (in the range of about 30% to about 80%).
A fourth embodiment of this invention relates to a radial ply runflat tire having a fabric overlay deployed radially outward of the belt structure in conjunction with the fabric underlay of the first embodiment.